In vitro model for neuronal death

ABSTRACT

This invention demonstrates the formation of a novel polarized membrane lipid raft signaling module in neurons, in response to several diverse neurotoxic stimuli. This polarization occurs well before neurons commit to die, and is an early mechanism in death signaling. The formation of this signaling module is dependent on cholesterol for its formation and provides a mechanistic explanation for the protective effects of cholesterol depleting drugs in several non-neural models of cell death. As such, the formation of the signaling module lends itself as a novel screen for the identification of new drugs and therapeutics which would retard its formation and protect against neuronal injury and death.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation application of U.S. patent application Ser. No.12/013,921, filed Jan. 14, 2008, which is a Divisional of U.S. patentapplication Ser. No. 10/989,918, filed Nov. 15, 2004, which claims thebenefit of U.S. Provisional Patent Application No. 60/520,285, filedNov. 14, 2003, which are hereby incorporated by reference in theirentireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not applicable.

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON COMPACT DISC

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention is neuronal death such as that which occursduring neurodegenerative diseases, and following stroke or trauma. Theinvention identifies a novel biochemical mechanism required to inducedeath in neurons from a variety of neurotoxic insults.

2. Description of the Related Art

Neuronal death underlies symptoms of neurodegenerative diseases (ND) anddamage from stroke and trauma. Neuronal death in neurodegenerativediseases such as Alzheimer's Disease (AD) and Parkinson's Disease (PD)is characterized by a number of features, including: (a) memory loss;(b) language deterioration; (c) impaired motor skills; (d) poorjudgment; and (e) indifferent attitude. Although AD usually begins afterage 60, its onset may occur as early as age 40. AD first appears asmemory decline. As the disease progresses over several years, cognition,personality, and the ability to function are all impaired or destroyed.

There is no cure for AD and no way to slow the progression of thedisease. For some people in the early or middle stages of the disease,medication such as tacrine may alleviate some cognitive symptoms. Also,some medications may help control behavioral symptoms such assleeplessness, agitation, wandering, anxiety. and depression. Thesetreatments are aimed solely at making the patient more comfortable anddo nothing to slow the progression of the underlying neuronal death inthe disease.

As such, there is much ongoing research that is aimed at theidentification and development of new therapeutic agents which can atleast slow, if not reverse, the death of neurons in AD and otherneurodegenerative diseases or following stroke or trauma.

An important tool in the identification of therapeutic agents forneuronal death would be the development of an in vitro model of neuronaldeath, which would replicate the biochemical steps involved in the deathprocess. Such an in vitro model would be invaluable for screening anddeveloping potential therapeutic agents.

As such, there is intense interest in the development of a practical invitro model for neuronal death, one which would exhibit biochemicalfeatures common to diverse stimuli.

In view of the intense interest in the development of practicalneurodegenerative disease models, a number of mouse models for AD and PDhave been developed to date. Despite the number of different mousemodels that have been developed, no one animal model demonstrates themarked neuronal death accepted as being completely correlative of thedegenerative disease condition. Thus, the biochemical mechanisms whichunderlie human ND disease and stroke related neuronal death are poorlyunderstood.

Therefore, there is great interest in the elucidation of the signaltransduction pathway which underlies neuronal death. This inventionidentifies a biochemical pathway used by all neurons in response todiverse neurotoxic stimuli. As such, it enables in vitro neuronal modelswhich demonstrate reliable, reproducible neuronal death to be used asscreens for compounds which would inhibit steps in the novel biochemicalpathway. Such a model would greatly facilitate the identifications ofcompounds which could be used to alleviate human neuronal disease.

Cells throughout the body are dependent upon the ability of membranereceptors to bind ligands and to effectively signal a cascade ofbiochemical events from the membrane to the nucleus. In the immunesystem, ligand binding to receptor (i.e. antigen to antigen receptor)induces changes in the morphology of the cell through cytoskeletalreorganization and induces the nucleus to activate the transcription ofnew genes to promote cellular differentiation and/or proliferation. Thischange in cell morphology is known as capping and refers to apolarization process in which cell surface proteins migrate to aspecific pole of the cell (Taylor et al., 1971). The molecular mechanismof capping is poorly understood. It is clear, however, that disruptionof the polarization process disrupts immune signaling (Bourguignon andBourguignon, 1984).

The plasma membrane of lymphoctes has recently been shown to containdiscrete lipid microdomains referred to as “lipid rafts” (Parton andSimons, 1995; Bromley et al., 2001). The terminology lipid raftsindicates microdomains of the plasma membrane which were firstidentified based on their insolubility in certain nonionic detergentsand are enriched in glycosphingolipids and cholesterol. This work hasshown that the plasma membrane is not a uniform lipid bilayer but ratherthat it contains specialized lipid microdomains which act as signalingplatforms for the transduction of external signals into cellularresponses. The aggregation of lipid rafts is essential for signaling incells throughout the body. Disruption of lipid raft aggregation throughthe use of inhibitors of lipid raft assembly (i.e. cholesteroldepleters) abrogates cell signaling. The relationship between cappingand lipid raft aggregation has been unclear but recent evidencedemonstrates that signal transduction involves the selective movement ofkey signaling proteins into and out of lipid rafts organizing asignaling module. In the immune system, this signaling module has beentermed the immunological synapse to describe the site of cell-cellcontact between communicating immune cells. Such spatial contact andcommunication (i.e. cell-cell co-capping) is essential for all immunefunctions. Khan et al., 2001 provided evidence for the polypeptide agrinas the first endogenous mediator of immune polarization and lipid raftaggregation. This was the first description of an endogenous lipid raftassembly inducing protein. The applicant reasoned that the polarizationof raft microdomains may be a conserved mechanism of biologicalsignaling used by neurons for signal transduction. This inventiondescribes the polarization of raft microdomains on the cell body/soma ofneurons in response to external stimuli. It demonstrates thatpolarization of raft microdomains is a signal transduction mechanismutilized by a variety of neurotoxic stimuli during death signaling.

SUMMARY OF THE INVENTION

The cause and the mechanism by which neurons die in neurodegenerativediseases are unknown. The mechanism by which neurons die followingstroke and trauma is also unknown.

Many damaged and dead neurons in autopsy brain sections from ND patientsshow condensed and fragmented chromatin, suggestive of apoptosis (aprogrammed cell death pathway). Further, treatment of cultured neuronsin vitro induces chromatin condensation and fragmentation alsosuggestive of apoptosis.

Brain ischemia leads to two forms of neuronal death. Necrotic neuronaldeath occurs in regions of severe blood flow insufficiency, while in thesurrounding regions (penumbra) where oxygen levels are higher and bloodflow is less compromised, apoptotic neuronal death is observed.Apoptotic death is indicated by condensed and fragmented chromatin,suggesting an orderly yet unknown death mechanism.

Neurodegenerative diseases such as Alzheimer's Disease (AD) andParkinson's Disease (PD) are characterized by marked neuronal death.Neuronal death also occurs following stroke and trauma, accounting forpost-injury symptoms such as paralysis and cognitive loss. The signaltransduction pathways involved in neuronal death in these situations arenot well understood. In non-neural cells such as lymphocytes,reorganization of membrane microdomains (lipid rafts) mediates formationof signaling complexes involved in transducing proliferative and deathsignals. The applicant the surface distribution of raft membranemicrodomains in in vitro cultures of cortical neurons after a variety ofdeath stimuli, by using the raft marker cholera toxin B (CTx-B) subunit,which binds the glycosphingolipid G_(M1). The applicant demonstratesthat diverse insults such as beta-amyloid peptide (Aβ), reactive oxygenspecies (1 mM H₂O₂), NMDA, neurotrophic factor deprivation, axotomy,mitochondrial poisons (arsenite), fas ligand crosslinking and hypoxiainduce rapid polarization of somal raft membranes. This polarizationoccurs well before neurons commit to die, consistent with a role as anearly mechanism in death signaling.

The applicant's data demonstrate that many diverse neuronal insultsinduce the formation of a polarized lipid raft mitochondria signalingmodule. The applicant's results show the formation of this signalingmodule is causally related to neuronal death, as inhibition of itsformation through a variety of drugs is protective against neurotoxicinsult. As such, formation of the signaling module lends itself as ascreen for the identification of new drugs and therapeutics, which wouldretard its generation and protect against neuronal injury and death.

An in vitro model for neuronal death, as well as methods for itspreparation and use, is provided. The in vitro model is characterized byneurons undergoing a shared common biochemical pathway, i.e. thepolarization of neuronal soma membrane lipid rafts, in response to avariety of death inducing stimuli. The in vitro model and mechanismfinds use in a important application, namely the screening of potentialtherapeutic agents for neurodegenerative diseases and stroke and trauma.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a further understanding of the nature, objects, and advantages ofthe present invention, reference should be had to the following detaileddescription, read in conjunction with the following drawings, whereinlike reference numerals denote like elements and wherein:

FIG. 1 panel A shows the distribution of lipid rafts in a restingcultured neuron. Raft distribution in clusters of cholera toxin Bstaining is uniformly distributed along the cell body surface. FIG. 1panel B shows the aggregation and polarization of raft microdomains to apole of the neuronal cell body following 15 min. exposure to 25 μMA-beta 25-35 peptide. Similar raft microdomain polarization is seenfollowing exposure to reactive oxygen species (1 mM H₂O₂), NMDA,neurotrophic factor deprivation, axotomy, mitochondrial poisons (e.g.,arsenite), fas ligand crosslinking, and hypoxia. FIG. 1 panel C showsthe inhibition of raft polarization following 15 min. exposure to 25 μMA-beta 25-35 peptide due to 1 hr pre-treatment with βCD, a cholesteroldepleting agent.

FIG. 2 shows staining for raft microdomains and A-beta peptide frombrain tissue sections of normal (A) and Alzheimer's Disease individuals.Arrows in FIG. 2, panel A indicate uniform distribution of cholera toxinB staining around cell bodies of neurons in normal brain control. Arrowsin FIG. 2, panel B indicate raft polarized microdomains co-stained andco-polarized with A-beta peptide.

FIG. 3 shows the protective effects as assessed by LDH release assay andchromatin fragmentation of compounds which inhibit lipid raftpolarization by lowering cholesterol levels in the neuronal membrane.Mevastatin, an HMG-CoA enzyme inhibitor and βCD, a cholesterol depletingagent, both inhibit lipid raft polarization and protect againstneurotoxic exposure from 25 μM A-beta 25-35 peptide and 300 μM NMDA.

FIG. 4A is a drawing and schematic of βCD, a cholesterol depletingagent, looking down on the cavity from above. FIG. 4B presents aperspective drawing of the 3-dimensional structure of γ-cyclodextrin.

FIG. 5 shows the uniform distribution of raft microdomains around thecell bodies of neuronal precursor cells (panel A) and CD34⁺ human stemcells (panel D). Proliferation following exposure to trophic factorsinduces polarization of raft microdomains in neuronal precursors (panelsB and C) and stem cells (panel E). Arrows indicate polarized raftmicrodomains in proliferating progenitors. Cholesterol depletioninhibits raft polarization and inhibits proliferation.

DETAILED DESCRIPTION OF THE INVENTION

The signal transduction pathways involved in neuronal death are not wellunderstood. In nonneural cells such as lymphocytes, reorganization ofmembrane microdomains (lipid rafts) mediates formation of signalingcomplexes involved in transducing proliferative and death signals. Thesurface distribution of raft membrane microdomains in cortical neuroncultures was examined after a variety of death stimuli, by using theraft marker cholera toxin B (CTx-B) subunit, which binds theglycosphingolipid G_(M1). This demonstrated that diverse insults such asbeta-amyloid peptide (Aβ), reactive oxygen species (1 mM H₂O₂), NMDA,neurotrophic factor deprivation, axotomy, mitochondrial poisons (e.g.,arsenite), fas ligand crosslinking, and hypoxia, induce rapidpolarization of somal raft membranes. An in vitro model for neuronaldeath, as well as methods for its preparation and use, is provided. Thein vitro model is characterized by exhibiting a novel biochemicalmechanism that is utilized by many neurotoxic insults to induce death.The in vitro model finds use in a variety of applications, includingscreening applications for potential neurodegenerative disease andstroke therapeutic agents. In further describing the subject invention,the in vitro model will be described first followed by a discussion ofrepresentative methods for its production and a review of applicationsin which the in vitro models find use.

Before the subject invention is further described, it is to beunderstood that the invention is not limited to the particularembodiments of the invention described below, as variations of theparticular embodiments may be made and still fall within the scope ofthe appended claims. It is also to be understood that the terminologyemployed is for the purpose of describing particular embodiments, and isnot intended to be limiting. Instead, the scope of the present inventionwill be established by the appended claims.

In this specification and the appended claims, the singular forms “a,”“an,” and “the” include plural reference unless the context clearlydictates otherwise. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as commonlyunderstood to one of ordinary skill in the art to which this inventionbelongs.

The subject invention provides a non-human animal model of neuronaldeath. A variety of different species of non-human animals areencompassed by the subject invention, where the subject animal neuronsin the model will typically be mammalian, including non-human primates,dogs, cats, cows, pigs, and the like, where species of the orderrodentia, e.g. mice, rats and guinea pigs, are of particular interest.Preferably, the subject non-human animal neurons in the model are mouse.

As the subject non-human in vitro models are ND animal models, theypreferably exhibit one or more characteristics analogous to the neuronaldeath characteristics seen in humans suffering from ND and stroke. Assuch, the subject animal neuronal models exhibit condensed or fragmentedchromatin.

The subject in vitro animal neurons in response to a variety ofneurotoxic insults such as beta-amyloid peptide (Aβ), reactive oxygenspecies (1 mM H₂O₂), NMDA, neurotrophic factor deprivation, axotomy,mitochondrial poisons (arsenite), fas ligand crosslinking and hypoxiahave polarized lipid raft microdomains. In other words, the neuronaltissue of the subject animals in response to neurotoxic agents ischaracterized by having established a polarization of somal membranelipid microdomains and established a polarized signaling platform. Thisis as compared to a control neurons e.g. unstimulated neurons culturedfrom a normal animal. By polarization of lipid raft microdomains it ismeant that the migration of lipid rafts and the proteins containedtherein to a pole of the neuronal cell body, the soma. In this fashion,the neuronal tissue of the subject animals has established a signalingplatform of lipid rafts and raft resident proteins to signal changes inthe neurons environment. Generally, the neuronal cells of the braintissue of the animal have a uniform distribution of lipid microdomainsaround the soma/cell body in resting neurons. Lipid raft microdomainsare also distributed in signaling clusters along axons and dendrites.These clusters are reduced following exposure to neurotoxic agents.Within the lipid rafts are raft resident proteins such asglycosylphosphatidyl-inositol-anchored proteins (GPI-linked) which canbe used as markers for lipid raft distribution. Lipid raft polarizationis mediated by changes in the actin microtubular cytoskeleton such thatactin and microtubule accumulate beneath the polarized raft complex andactin/microtubule detecting probes can also be used to mark thepolarization of raft microdomains and are part of the somal polarizationresponse. Mitochondria also re-organize beneath the polarized raftmicrodomains and may be used to monitor the polarization of raftmicrodomians and are part of the neuronal cell body response toneurotoxic stimuli. Dyes, such as the styryl dye FM1-43 may also be usedto monitor neuronal raft polarization. FM1-43 is an amphipathic moleculethat intercalates spontaneously into the outer leaflet of cell membraneswithout diffusing across the membrane. Within the lipid environment,FM1-43 exhibits 50- to 100-fold increased fluorescence intensity thanthe dye in aqueous solution. FM1-43 fluorescence increases in thepolarized raft complex, and an increase in fluorescence can therefore beused as an index of raft polarization dynamics. Clearly, there are manyways to monitor the distribution of lipid rafts in the neuronalmembrane. Probes labeled for detection which recognize sugars andgangliosides such as GM1 (cholera toxin subunit B), antibodies toproteins particular to lipid rafts (GPI-linked etc.) or lipidsparticular to lipid rafts, dyes which incorporate predominantly intoraft domains (FM1-43) are all incorporated herein as a means to detectthe distribution of raft microdomains on neuronal membranes. Manyproteins are modified by fatty acid acylation with myristate orpalmitate. These and such modifications help target these proteins toraft microdomains and may also be used to track the distribution of raftmicrodomains. The novelty of the invention disclosed herein is thepolarization of neuronal soma raft microdomains as a signal transductionmechanism in response to external stimuli. The means to detect raftmicrodomain distribution are well known to those skilled in the art andare incorporated herein.

A critical feature of the neuronal in vitro model is that the neuronaltissue in response to neurotoxic agents is characterized by a uniformconserved biochemical mechanism which is utilized by all neurons forsignal transduction. In other words, lipid raft polarization is a commonbiochemical mechanism utilized by all neurons as a part of their signaltransduction response to toxic agent exposure. The polarization andestablishment of a signaling platform in response to neurotoxin exposureis the same for all neurotoxins and is observed in the autopsy brains ofhuman subjects suffering from neuronal death and degeneration in AD andPD.

Therefore, the mechanism of neuronal death in the subject animalsneuronal in vitro model is substantially similar to the lipid raftpolarization mechanism observed in human AD and PD Disease subjects.

Generally, the subject animals will be embryonic animals, where byembryonic is meant an in utero animal that comprises an ability to haveits neurons cultured in vitro. Typically, this neuronal cultureprocedure utilizes embryonic day 17 in utero pups as a source ofneuronal tissue. Currently, cultured neurons are not possible from adultanimals but this model would apply to such neurons if culture techniquesimprove.

The in vitro model is also applicable to cultured neurons fromtransgenic animals carrying any transgene.

In any embodiment, the transgene will be a transgene which is capable ofbeing expressed in the brain tissue of the animal.

Example 1 Methods for Producing the Subject In Vitro Neuronal Model

Numerous descriptions of protocols for preparing neurons are publishedand known to those of skill in the art. As such, methods for productionof such neurons are readily practiced by those of skill in the art.

Briefly, cortical cultures enriched in cells of neuronal lineage can beprepared from 16-day mouse embryos with Neurobasal medium containing B27supplement, 2 mM glutamate, and 1% penicillin and streptomycin. After 4days, one-half of the medium is replaced with an appropriate medium suchas Neurobasal medium containing 2% B27 growth supplement, andexperiments can be conducted at 8 days in vitro a sufficient time forneurons to grow and differentiate.

The animal in vitro model neurons are then given an effective amount ofa neurotoxic stimulus to induce neuronal death. The lipid raftpolarization inhibitor may be any compound that inhibits or blocks theformation of the raft microdomain signaling complex. As such. the raftpolarization inhibitor may be: an inhibitor of cholesterol synthesissuch as HMG-CoA reductase inhibitors (statins) and the like; or an acutedepleter of cholesterol, such as methyl-β-cyclodextrin (βCD).

Linking D-gluocse units together with α-1,4 linkages means that thegrowth of the polysaccharide follows a helical path (FIGS. 4A & 4B).Occasionally, this coiling brings the D-glucose at the end of thegrowing polymer chain close enough to the one at the beginning that aglycosidic bond can form between them, thereby creating a cyclicpolysaccharide. These structures are known as cyclodextrins. FIG. 4Apresents the structure of one such compound which contains a ringcomprised of eight D-glucose units. This compound is known asγ-cyclodextrin. Cyclodextrins are natural products formed by the actionof enzymes called cycloglucosyltransferases, CGTases, on starch. Theseenzymes are found in a microorganism called Bacillus macerans.Cyclodextrins participate in host-guest interactions, serving as hostsfor a variety of small molecules. The number of monomer units in themacrocyclic ring determines the size of the cavity the host makesavailable to the guest.

The ability of cyclodextrins to “encapsulate” small molecules has led totheir use as cholesterol depletors and disrupters of lipid rafts incells and neurons.

In FIG. 4A, you are looking down on the cavity from above. FIG. 4Bpresents a perspective drawing of the 3-dimensional structure ofγ-cyclodextrin. The conformation of the glucose units in thecyclodextrin places the hydrophilic hydroxyl groups at the top andbottom of the three dimensional ring and the hydrophobic glycosidicgroups on the interior. Note that the polar OH groups project to theexterior of the structure while the hydrogens attached to the glucoseunits point into the cavity. Thus the interior is comparativelynon-polar. These structural features make the polymer water solublewhile still able to transport non-polar materials such as cholesterol.When cyclodextrin is applied to cells or neurons cholesterol is depletedfrom cellular membranes and resides within the interior non-polarcavity. The depletion of cholesterol from cell membranes disrupts lipidrafts and inhibits cell signaling through raft domains.

Example 2 Methods of Using the Subject In Vitro Neuronal Model

The neuronal in vitro animal models exhibiting polarization of lipidmicrodomains find use in a variety of applications particularly inresearch applications, including research applications designed toelucidate the role of various genes and or agents in the development orprogression of ND as well as in research applications designed toidentify therapeutic agents for the treatment or amelioration ofneurodegenerative disease and stroke symptoms.

For example, as the in vitro neuronal model demonstrates a biochemicalmechanism analogous to those observed in human disease subjects, themodel can be used to study the effect of various genes and theirexpression products in the development of neurodegenerative disease.Thus, the subject in vitro model is suitable for elucidation ofcompounds that modulate the progression of neurodegenerative disease.

Of particular interest is the use of the in vitro model for thescreening of potential ND therapeutic agents. Through use of the model,one can identify compounds that modulate the progression of ND, e.g., bybinding to, modulating, enhancing or repressing the activity of aprotein or peptide involved in the progression of ND, e.g. a neurotoxicAβ peptide. Screening to determine drugs that lack effect on theprogression of ND is also of interest. Areas of investigation are thedevelopment of anti-degenerative or cognitive therapies. Of particularinterest are screening assays for agents that have a low toxicity forhuman cells. Assays of the invention make it possible to identifycompounds which ultimately (1) have a positive affect with respect to NDprogression and as such are therapeutics, e.g. agents which arrest orreverse the neurodegeneration and or memory/learning deteriorationassociated with ND; or (2) have an adverse affect with respect to NDprogression and as such should be avoided as therapeutic agents and inproducts consumed by animals, in particular humans.

A wide variety of assays may be used for this purpose, including in vivobehavioral studies, determination of the localization of drugs afteradministration, labeled in vitro protein-protein binding assays,protein-DNA binding assays, electrophoretic mobility shift assays,immunoassays for protein binding, and the like. Depending on theparticular assay, whole animals may be used, or cells derived therefrom.Cells may be freshly isolated from an animal, or may be immortalized inculture. Cells of particular interest include neural and brain tissue ofanimals.

The term “agent” as used herein describes any molecule, e.g. protein ornonprotein organic pharmaceutical, with the capability of affecting thepolarization of lipid microdomains underlying neuronal death. Generallya plurality of assay mixtures are run in parallel with different agentconcentrations to obtain a differential response to the variousconcentrations. Typically, one of these concentrations serves as anegative control, i.e. at zero concentration or below the level ofdetection.

Candidate agents encompass numerous chemical classes, though typicallythey are organic molecules, preferably small organic compounds having amolecular weight of more than 50 and less than about 2,500 daltons.Candidate agents comprise functional groups necessary for structuralinteraction with proteins, particularly hydrogen bonding, and typicallyinclude at least an amine, carbonyl, hydroxyl or carboxyl group,preferably at least two of the functional chemical groups. The candidateagents often comprise cyclical carbon or heterocyclic structures andloraromatic or polyaromatic structures substituted with one or more of theabove functional groups. Candidate agents are also found amongbiomolecules including, but not limited to: peptides, saccharides, fattyacids, steroids, purines, pyrimidines, derivatives, structural analogsor combinations thereof.

Candidate agents are obtained from a wide variety of sources includinglibraries of synthetic or natural compounds. For example, numerous meansare available for random and directed synthesis of a wide variety oforganic compounds and biomolecules, including expression of randomizedoligonucleotides and oligopeptides. Alternatively, libraries of naturalcompounds in the form of bacterial. fungal, plant and animal extractsare available or readily produced. Additionally, natural orsynthetically produced libraries and compounds are readily modifiedthrough conventional chemical, physical and biochemical means, and maybe used to produce combinatorial libraries. Known pharmacological agentsmay be subjected to directed or random chemical modifications, such asacylation, alkylation, esterification, amidification, etc. to producestructural analogs. New potential therapeutic agents may also be createdusing methods such as rational drug design or computer modeling.

Screening may be directed to known pharmacologically active compoundsand chemical analogs thereof, or to new agents with unknown propertiessuch as those created through rational drug design. Candidate agents forarresting and or reversing raft microdomain polarization/neuron raftremodeling, or recovery from acute insults to the nervous system can bescreened for their ability to modulate ND progression. Efficaciouscandidates can be identified by arrest or reversal of raft microdomainpolarization in comparison with untreated control neurons. In anotherexample, a candidate agent for the treatment of impaired learningfunction attributable to AD can be identified by an inhibition of raftmicrodomain polarization following administration of the candidateagent. Alternatively, candidate agents may be identified by theirability to modulate the activity of a protein implicated in ND, andassociated with raft microdomains.

Where the screening assay is a binding assay, one or more of themolecules may be joined to a label, where the label can directly orindirectly provide a detectable signal.

Various labels include radioisotopes, fluorescent or chemiluminescentmoieties, enzymes, specific binding molecules, particles, e.g. magneticparticles, and the like. Specific binding molecules include pairs, suchas biotin and streptavidin, digoxin and antidigoxin, etc. For thespecific binding members, the complementary member would normally belabeled with a molecule that provides for detection, in accordance withknown procedures. A variety of other reagents may be included in thescreening assay. These include reagents like salts, neutral proteins,e.g. albumin, detergents, etc that are used to facilitate optimalprotein-protein binding andlor reduce non-specific or backgroundinteractions. Reagents that improve the efficiency of the assay, such asprotease inhibitors, nuclease inhibitors, anti-microbial agents, etc.may be used. The mixture of components are added in any order thatprovides for the requisite binding. Incubations are performed at anysuitable temperature, typically between 4 and 40° C. Incubation periodsare selected for optimum activity, but may also be optimized tofacilitate rapid high-throughput screening. Typically between 0.1 and 1hours will be sufficient.

Samples, as used herein, include biological fluids such as tracheallavage, blood, cerebrospinal fluid, tears, saliva, lymph, dialysis fluidand the like; organ or tissue culture derived fluids; and fluidsextracted from physiological tissues. Also included in the term arederivatives and fractions of such fluids. The number of cells in asample will generally be at least about 10³, usually at least 10⁴ moreusually at least about 10⁵. The cells may be dissociated, in the case ofsolid tissues, or tissue sections may be analyzed.

Alternatively a lysate of the cells may be prepared. For example,detection may utilize staining of cells or histological sections,performed in accordance with conventional methods. The antibodies ofinterest are added to the cell sample, and incubated for a period oftime sufficient to allow binding to the epitope, usually at least about10 minutes. The antibody may be labeled with radioisotopes, enzymes,fluorescers, chemiluminescers, or other labels for direct detection.Alternatively, a second stage antibody or reagent is used to amplify thesignal. Such reagents are well known in the art. For example, theprimary antibody may be conjugated to biotin, with horseradishperoxidase-conjugated avidin added as a second stage reagent. Finaldetection uses a substrate that undergoes a color change in the presenceof the peroxidase. The absence or presence of antibody binding may bedetermined by various methods, including flow cytometry of dissociatedcells, microscopy, radiography, scintillation counting, etc.

An alternative method depends on the in vitro detection of bindingbetween antibodies and a protein of interest in a lysate. Measuring theconcentration of binding in a sample or fraction thereof may beaccomplished by a variety of specific assays. A conventional sandwichtype assay may be used. For example, a sandwich assay may first attachspecific antibodies to an insoluble surface or support. The particularmanner of binding is not crucial so long as it is compatible with thereagents and overall methods of the invention. They may be bound to theplates covalently or non-covalently, preferably non-covalently.

The insoluble supports may be any compositions to which polypeptides canbe bound, which is readily separated from soluble material, and which isotherwise compatible with the overall method. The surface of suchsupports may be solid or porous and of any convenient shape. Examples ofsuitable insoluble supports to which the receptor is bound includebeads, e.g. magnetic beads, membranes and microtiter plates. These aretypically made of glass, plastic (e.g. polystyrene), polysaccharides,nylon or nitrocellulose. Microtiter plates are especially convenientbecause a large number of assays can be carried out simultaneously,using small amounts of reagents and samples. A number of assays areknown in the art for determining the effect of a drug on animal behaviorand other phenomena associated with neurodegeneration or impairment ofcognitive abilities as observed in AD. Some examples are provided,although it will be understood by one of skill in the art that manyother assays may also be used. The subject animals may be used bythemselves, or in combination with control animals.

The screen and candidate molecules identified therein using the in vitromodel of the invention can employ any phenomena associated learningimpairment, dementia or cognitive disorders that can be readily assessedin an animal model. The screening can include assessment of phenomenaincluding, but not limited to: 1) analysis of molecular markers (e.g.,levels of expression of particular gene product in brain tissue;presence/absence in brain tissue of particular protein isoforms; andformation of neurite plaques); 2) assessment behavioral symptomsassociated with memory and learning; 3) detection of neurodegenerationcharacterized by progressive and irreversible deafferentation of thelimbic system, association neocortex, and basal forebrain(neurodegeneration can be measured by, for example, detection ofsynaptophysin expression in brain tissue). These phenomena may beassessed in the screening assays either singly or in any combination.

Preferably the screen will include control values (e.g., the dispositionof raft microdomains in the test animal in the absence of testcompound(s)). Test substances which are considered positive, i.e.,likely to be beneficial in the treatment of ND are those which reverseor inhibit raft polarization in in vitro model screen.

Methods for assessing these phenomena, and the effects expected of acandidate agent for treatment of ND, are known in the art. For example,methods for using transgenic animals in various screening assays fortesting compounds for an effect on ND, are found in WO 9640896,published Dec. 19, 1996.

Example 3 Analysis of Gene Expression

1) mRNA: mRNA can be isolated by the acid guanidiniumthiocvanatephenol:chloroform extraction method (Chomczvnski et al., AnalBiochem. 162:156-159 (1987)) from cell lines and tissues of transgenicanimals to determine expression levels by Northern blots.

2) In situ Hybridizations: Radioactive or enzymatically labeled probescan be used to detect mRNA in situ. The probes are degradedapproximately to 100 nucleotides in length for better penetration ofcells. The procedure of Chou et al. J Psychiatr Res. 24:27-50 (1990) forfixed and paraffin embedded samples is briefly described below althoughsimilar procedures can be employed with samples sectioned as frozenmaterial. Paraffin slides for in situ hybridization are dewaxed inxylene and rehydrated in a graded series of ethanols and finally rinsedin phosphate buffered saline (PBS). The sections are postfixed in fresh4% paraformaldehyde. The slides are washed with PBS twice for 5 minutesto remove paraformaldehyde. Then the sections are permeabilized bytreatment with a 20 mu g/ml proteinase K solution. The sections arerefixed in 4% paraformaldehyde, and basic molecules that could give riseto background probe binding are acetylated in a 0.1M triethanolamine,0.3M acetic anhydride solution for 10 minutes. The slides are washed inPBS, then dehydrated in a graded series of ethanols and air dried.Sections are hybridized with antisense probe, using sense probe as acontrol. After appropriate washing, bound radioactive probes aredetected by autoradiography or enzymatically labeled probes are detectedthrough reaction with the appropriate chromogenic substrates.

3) Western Blot Analysis: Protein fractions can be isolated from tissuehomogenenates and cell lysates and subjected to Western blot analysis asdescribed by Harlow et al., Antibodies: A laboratory manual, Cold SpringHarbor, N.Y., (1988); Brown et al., J. Neurochem 40:299-308 (1983); andTate-Ostroff et al. Proc Natl Acad Sci 86:745-749 (1989)). Only a briefdescription is given below. The protein fractions can be denatured inLaemmli sample buffer and electrophoresed on SDS-polyacrylamide gels.The proteins are be then transferred to nitrocellulose filters byelectroblotting. The filters are blocked, incubated with primaryantibodies, and finally reacted with enzyme conjugated secondaryantibodies. Subsequent incubation with the appropriate chromogenicsubstrate reveals the position of proteins.

Example 4 Therapeutic Agents Identified with the In Vitro Model

This invention describes lipid raft polarization and establishment of apolarized raft signaling platform in neurons exposed to a variety ofneuroxic insults ranging from beta-amyloid peptide (Aβ), reactive oxygenspecies (1 mM H₂O₂), NMDA, neurotrophic factor deprivation, axotomy,arsenite, fas ligand crosslinking and hypoxia. The range of neurotoxicinsults is wide and is meant to indicate the diversity of neurotoxicinsults which use lipid raft polarization to sense and mount a responseto the insult. As such polarization of neuronal raft microdomains is anintegral part of the neurons signal transduction pathway. Importantly,the raft polarization is an early part of the neurons biochemicalresponse and as such very amenable to therapeutic intervention whichwould block or retard the death process.

The HMG-CoA reductase enzyme is rate limiting in cholesterolbiosynthesis. Pre-treatment of cortical cultures with HMG-CoA reductaseinhibitors (statins) or acute treatment with the cholesterol-extractingagent β-cyclodextrin inhibits lipid raft polarization and protectsagainst neuronal death. This mode of depleting cholesterol to inhibitraft microdomain polarization has been used in lymphocytes to disruptimmune signaling. Neuroprotection is substantially attenuated byco-treatment with either mevalonate or cholesterol and is mimicked byacute treatment with the cholesterol-extracting agent β-cyclodextrin,indicating that neuroprotection is mediated by depletion of a cellularpool of cholesterol because of the inhibition of cholesterolbiosynthesis. Mevalonate acts as a precursor of not only cholesterol butalso isoprenoids for farnesyl and geranylgeranyl molecules, which havean important signaling function.

Example 5

Cholesterol depletion inhibits lipid raft polarization and protectscortical neurons from AB peptide and NMDA-induced neuronal death asmeasured by LDH release and chromatin fragmentation.

Cultured cortical neurons at 8 DIV were untreated (control group, CNTL),treated with 20 μM Aβ 25-35 for 48 hr (A-Beta group) or exposed to 300μM NMDA/5 μM glycine for 10 min (NMDA group), as shown in FIG. 3.Neuronal cultures were also pre-treated chronically with 300 nMmevastatin for five days (cholesterol depleted group, chol_(meva)), oracutely with 1% βCD for 1 hr (cholesterol depleted group, chol_(βCD))before 20 μM Aβ 25-35 treatment or 300 nM NMDA treatment. At the end of48 h of 20 μM Aβ 25-35 treatment and 24 h following 300 nM NMDAexposure, culture medium was collected for LDH release assay.Cholesterol depletion induced resistance to 20 μM Aβ 25-35 or 300 nMNMDA induced neuronal death was prevented by co-incubation of 300 nMmevastatin with 5 μM cholesterol (cholesterol depleted group,Meva_(chol), (A-Beta)/(NMDA)) or a 15 min treatment with 2 mmcholesterol complexed with 2.5% βCD to replace cholesterol followingacute depletion with 1% βCD (cholesterol depleted group, βCD_(chol),(Aβ)/(NMDA)). The LDH release value obtained from cortical neuronsexposed to 1% Triton X-100-containing lysis buffer served as 100% LDHrelease, and data obtained in other groups were calculated as percent ofthis value accordingly. Data are expressed as the mean±S.E.M (n=3).Control and cholesterol depleted cortical neurons were assessed forchromatin integrity (DAPI) and CTx-B staining distribution following 20μM Aβ 25-35 and 300 nM NMDA treatment. We determined the percentage ofneurons with polarized CTx-B staining and the percent nuclei withcondensed and fragmented chromatin for each experimental group ofneurons by counting 500 neurons in several random fields. Three hoursfollowing 20 μM Aβ 25-35 or 1 h following 300 μM NMDA treatment, thepercentage of neurons with polarized raft microdomains is significantlylower and 48 h following 20 μM Aβ 25-35 or 24 h following 300 μM NMDAtreatment the percentage of neurons with condensed and fragmentedchromatin is significantly lower in cholesterol-depleted neurons.

The 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG-CoA) reductaseinhibitors (statins) reduce synthesis of cholesterol and isoprenoids,molecules that modulate diverse cell function. Inhibition of the enzymeHMG-CoA reductase depletes downstream isoprenoids such as geranylgeranylpyrophosphate and farnesyl pyrophosphate. These isoprenoids not onlyserve as intermediates for cholesterol biosynthesis but modify proteinsto facilitate their attachment to cell membranes. For example, HMG-CoAinhibition blocks geranylgeranylation of G-proteins such as Rho GTPases,thereby inhibiting GTPase activity and causing disruption of actinstress fibers. Thus, raft microdomain polarization and neuronal death isblocked by cholesterol depletion, rho GTPase inhibition, or inhibitionof geranyl and farnesyl membrane attachment moieties. These differentmodes of raft polarization are targets for therapeutic intervention.

All statins inhibit polarization of lipid microdomains in vitro and areprotective against neurotoxic insults. However, all statins may not beequipotent in protecting the brain against neurotoxic insults in vivo.The differences between statins may relate to variable drug penetrationinto the brain based on differences in lipophilicity. For example, theactive form of mevastatin is approximately 8 times less lipophilic thansimvastatin, and lovastatin is less lipophilic than simvastatin. Hence,statins with the highest lipophilicity and potency may provide thegreatest degree of neuro-protection.

Filipin is a polyene antibiotic that specifically complexes cholesteroland disrupts lipid raft function.

Rho kinase inhibitor Y-27632.

Pre-treatment of cortical cultures with cytochalasin D, a drug thatprevents actin polymerization, prevents the polarization of lipidmicrodomains.

Pre-treatment of cortical cultures with cytochalasin D or latrunculins,drugs that prevents actin polymerization, prevents the polarization oflipid microdomains.

The HMG-CoA reductase enzyme is rate limiting in cholesterolbiosynthesis. HMG-CoA reductase inhibitors protect cortical neurons fromNMDA mediated excitotoxicity and reduce infarct volume after cerebralischemia in mice. Pre-treatment of cortical cultures with HMG-CoAreductase inhibitors (statins) or acute treatment with thecholesterol-extracting agent β-cyclodextrin inhibits lipid raftpolarization and protects against neuronal death. Neuroprotection issubstantially attenuated by co-treatment with either mevalonate orcholesterol and is mimicked by acute treatment with thecholesterol-extracting agent β-cyclodextrin, suggesting thatneuroprotection is mediated by depletion of a cellular pool ofcholesterol because of the inhibition of cholesterol biosynthesis.

Polyunsaturated fatty acids (PUFA) n-3 inhibit raft microdomainpolarization. PUFA n-3 are more effective than PUFA n-6 in inhibitingraft polarization following neurotoxic exposure.

n-Butyrate,carnitine and related compounds also inhibit raftpolarization.

Selected analogues of propranolol (alprenolol, oxprenolol, metoprolol,practolol, and sotalol) calmodulin inhibitors phenothiazine-drugs whoseactivities can be differentiated as follows: trifluoperazine greaterthan chlorpromazine greater than sulfoxide derivativesimmunosuppressants FK-506 rapamycin cyclosporine

Ether-linked lysophospholipids and alkylphosphocholines anti-tumor etherlipids ET-18-OCH₃ and hexadecylphosphocholine

Erythropoietin

Inhibitors of Mek (Map/Erk) Kinases

Neuronal cultures derived from embryonic precursors contain neuronalprecursors which in response to trophic factors divide and differentiateinto neurons. Similar proliferative events occur in in vitro human stemcell division whereby stem cells divide before entering terminaldifferentiation. The process of proliferation before finaldifferentiation also utilizes specific signal transduction mechanisms.These proliferative events in neuronal progenitor cells and human stemcells both utilize polarization of raft microdomains to organizesignaling elements at a specific pole of the cell body. Similarconservation of cytoarchitectual form is seen in the immune systemwhereby lymphocytes use a polarized cap structure to signal bothproliferation and death. As such, the formation of this signalingcomplex for proliferation of neuronal progenitors and stem cells is alsosensitive to cholesterol depleting drugs and drugs which impair theformation of the death inducing raft microdomain. Drugs which impairformation of raft polarization could enhance terminal differentiation.

The in vitro model can be used to identify additional therapeutic agentsfor the treatment and/or amelioration of neuronal death, e.g. for use inslowing or reversing neurodegeneration and/or the degeneration oflearning/memory abilities. By treatment is meant at least a reduction inan neurodegenerative disease or stroke parameter. such as a slowing inthe progression of the disease, a reduction in the magnitude of asymptom thereof, e.g. a reduction in the level of learning impairment.and the like.

With neurodegenerative disease and stroke/trauma patients, thetherapeutic agents identified using the subject animal models may beadministered in a variety of ways, such as orally. topically.parenterally e.g. subcutaneously. intraperitoneally, by viral infection.intravascularly. etc. Inhaled treatments are of particular interest.Depending upon the manner of introduction, the compounds may beformulated in a variety of ways. The concentration of therapeuticallyactive compound in the formulation may vary from about 0.1-100 wt. %.

The pharmaceutical compositions can be prepared in various forms. suchas granules, tablets, pills, suppositories, capsules, suspensions,salves, lotions and the like. Pharmaceutical grade organic or inorganiccarriers and/or diluents suitable for oral and topical use can be usedto make up compositions containing the therapeutically-active compounds.Diluents known to the art include aqueous media, vegetable and animaloils and fats. Stabilizing agents, wetting and emulsifying agents, saltsfor varying the osmotic pressure or buffers for securing an adequate pHvalue, and skin penetration enhancers can be used as auxiliary agents.

The foregoing examples are offered by way of illustration and not by wayof limitation. It is apparent from the above results and discussion thatthe identification of a novel neuronal biochemical mechanism is providedby the subject invention. In this model, neurons exposed to neurotoxicstimuli are characterized by the polarization of raft microdomainsanalogous to that observed in human ND patients, providing for an animalmodel that more closely correlates to human disease and injury ascompared to other currently known neuronal models.

As such, the subject invention provides for a significant advance in theart.

All publications and patent applications cited in this specification areherein incorporated by reference as if each individual publication orpatent application were specifically and individually indicated to beincorporated by reference. The citation of any publication is for itsdisclosure prior to the filing date and should not be construed as anadmission that the present invention is not entitled to antedate suchpublication by virtue of prior invention.

It will be understood that each of the elements described above, or twoor more together may also find a useful application in other types ofmethods differing from the type described above. Without furtheranalysis, the foregoing will so fully reveal the gist of the presentinvention that others can, by applying current knowledge, readily adaptit for various applications without omitting features that, from thestandpoint of prior art, fairly constitute essential characteristics ofthe generic or specific aspects of this invention set forth in theappended claims. The foregoing embodiments are presented by way ofexample only; the scope of the present invention is to be limited onlyby the following claims.

The invention claimed is:
 1. A population of cholesterol-depletedmammalian stem cells in vitro, produced by the process comprising: (a)culturing a starting population comprising mammalian stem cells in acell culture medium; (b) amplifying and/or enriching for saidcholesterol-depleted mammalian stem cells by adding at least about 0.1%cyclodextrin to the cell culture medium in which said startingpopulation is cultured.
 2. The population of cholesterol-depletedmammalian stem cells of claim 1, wherein the cyclodextrin ismethyl-β-cyclodextrin.
 3. The population of cholesterol-depletedmammalian stem cells in vitro of claim 1, wherein said startingpopulation remains in contact with said cell culture medium comprisingat least about 0.1% cyclodextrin for about 1 hour to about 49 hours. 4.The population of cholesterol-depleted mammalian stem cells of claim 1,wherein said stem cells are resistant to insult as measured by a methodfrom the group consisting of LDH release assay and chromatinfragmentation.
 5. The population of cholesterol-depleted mammalian stemcells in vitro of claim 1, wherein the added cyclodextrin is at least 1%cyclodextrin.
 6. The population of cholesterol-depleted mammalian stemcells in vitro of claim 1, wherein the cyclodextrin is betacyclodextrin.
 7. The population of cholesterol-depleted mammalian stemcells in vitro of claim 1, wherein said stem cells are human.
 8. Amethod of producing a population of cholesterol-depleted mammalian stemcells, the method comprising administering at least about 0.1%cyclodextrin to the growth medium in which a starting populationcomprising mammalian stem cells in vitro is cultured.
 9. The method ofclaim 8, wherein the cyclodextrin is methyl-β-cyclodextrin.
 10. Themethod of claim 8, wherein said starting population comprising mammaliancells in vitro remains in contact with said growth medium, to which saidat least about 0.1% cyclodextrin has been added, for about 1 hour toabout 49 hours.
 11. A population of mammalian stem cells havinginhibited lipid raft polarization in vitro, produced by the processcomprising: (a) culturing a starting population comprising mammalianstem cells in a cell culture medium; (b) amplifying and/or enriching forsaid mammalian stem cells having inhibited lipid raft polarization byadding at least about 0.1% cyclodextrin to the cell culture medium inwhich said starting population is cultured.
 12. The population ofmammalian stem cells having inhibited lipid raft polarization of claim11, wherein the cyclodextrin is methyl-β-cyclodextrin.
 13. Thepopulation of mammalian stem cells having inhibited lipid raftpolarization in vitro of claim 11, wherein said starting populationremains in contact with said cell culture medium comprising at leastabout 0.1% cyclodextrin for about 1 hour to about 49 hours.